Kinetics of NO reduction by CO on quartz glass surfaces

UTTERWORTH EINEMANN

Fuel Vol 74 No. 3, pp. 452-455, 1995 Copyright 0 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0016-2361/95/.$10.00+0.00

Kinetics of NO reduction glass surfaces Andreas

Berger

by CO on quartz

and Gerd Rotzoll

lnstitut fiir Technische Chemie, Universitgt Federal Republic of Germany (Received 6 June 7994)

Hannover,

Callinstr. 3, 30 167 Hannover,

The reduction of NO by CO was studied in a tubular reactor filled with small pieces of quartz glass tubing. The influence of residence time, concentration of CO and temperature on the reaction was investigated. The reaction rate is first-order with respect to both NO and CO. The rate constant, determined over the temperature range 973-l 123K and for a total surface-to-volume ratio of the filled reactor of 23 cm- l, is k = 5.4.107 exp( - 2800/T) cm3 s- ’mol- *. Comparison with the reaction rate over char indicates that the overall effect of sand may be of similar magnitude to that of char for a typical solids mixture composition in fluidized beds. (Keywords nitric oxide; reduction; carbon monoxide)

The amount of NO emitted from fluidized-bed combustors is the result of homogeneous and heterogeneous reactions forming and consuming NO. To gain a deeper understanding of the actual reactions taking place and to be able to perform model calculations, it is necessary to identify the most important reactions and measure their kinetics. When coal is burnt in fluidized-bed combustors, the heterogeneous reduction of NO at the char surface is presumably one of the most important reactions removing NO. Its kinetics have been investigated many timesle9, and a promoting effect of CO has been established. This can be explained by the following mechanism2v4: NO dissociates after adsorption on the char surface into chemisorbed N and 0 atoms. Whereas the N atoms quickly recombine to N, and leave the surface, the 0 atoms are strongly chemisorbed and can be removed from the surface to produce CO only. Gas-phase CO enhances the rate of NO reduction by reacting with and removing surface-bound 0 atoms as C02, thus regenerating surface sites for new adsorption of NO. In this case the char surface acts as a catalyst only. Since the solids in a fluidized bed consist of various materials such as sand, limestone, ash and char, the question arises whether the char, which makes up only a minor portion of the bed material, is alone able to catalyse the NO-CO reaction. The reaction might also be catalysed by the other constituents of the bed. Indeed, reactions between NO and CO which were interpreted as wall reactions were observed quite early2. Systematic kinetic studies have been made of the reduction of NO by CO catalysed by CaO”, according to which the catalytic activity of this material is even higher than that of char. Even simultaneous reaction with SO, does not lower the activity”. It is often denied3v6 that sand, the main constituent of many fluidized beds, has a catalytic activity. There is

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evidence from concentration profile measurements in fluidized beds, however, that the NO-CO reaction is catalysed by sand or quartz glass surfaces too12. The reaction becomes noticeable only at CO concentrations of > 1%. Such high CO concentrations usually occur only under reducing conditions, for example with bad mixing or incomplete air supply in staged combustion. However, the reaction could also take place with excess oxygen. Recent experiments13 with sand beds fluidized by mixtures of CO with air have revealed that such mixtures apparently do not burn up to temperatures of - 1000°C in the particulate phase, but only in the bubbles and in the freeboard above the bed surface. If this is so and the main or sole primary product of char oxidation is CO, this would mean that CO and 0, could coexist in the particulate phase and that the NO-CO reaction catalysed by sand could even take place in the presence of oxygen. The question of how important compared with the other bed materials sand is in catalysing the NO-CO reaction can only be answered by model calculations. For this purpose it is necessary to provide quantitative kinetic data on the reaction, which is the object of this

paper. EXPERIMENTAL The kinetic experiments were performed in a tubular reactor of quartz glass, consisting of three sections: a preheating section 36 cm long and 0.6 cm i.d., a reaction section 19 cm long and 2.4 cm i.d. and a quench section with indirect water cooling. The reaction section was filled with short pieces of quartz glass tubing, each 0.5 cm o.d. and 0.3 cm long, to provide a greater surface for reaction. It is assumed that quartz glass is similar to sand in catalytic activity. The total surface-to-volume ratio (S/V) of the filled reactor, i.e. the ratio of the total surface

Kinetics of NO reduction

area including the filling to the free reactor volume, was 23 cm-‘. The reactor was heated by an electrical three-zone tube furnace. The reaction section was located in the centre of the furnace, where the furnace temperature was almost uniform. The preheating section was partly outside and partly inside the furnace in the zone of increasing temperature. Axial gas temperature profiles in the reactor were measured with a thermocouple. In the first part of the reaction section the temperature was almost uniform, but it decreased somewhat towards the end as a consequence of the water cooling. This was of little consequence, as the temperature dependence of the reaction rate is weak, as will be shown below. The temperatures quoted refer to the almost constant values in the first part of the reaction section. Residence times T were calculated as T= I//F,, I/ being the free reactor volume and F, the volumetric gas flow rate. Gas mixtures were prepared and flow rates adjusted by three mass flow controllers. The main component was N,, with variable amounts of NO and CO, at a total pressure of 100 kPa. Gas composition was determined by an infrared analyser for CO and a chemiluminescence analyser for NO. For each specific condition of temperature and gas concentrations, the gas flow through the reactor was maintained until the exit concentrations had reached stationary values. This could take up to 3 h, indicating slow change in the surface conditions during reaction. The experimental programme covered the following ranges of variables: residence time 0.5-l s; NO concentration 65&1000 ppmv; CO concentration OS-5 vol.%; temperature 973-l 173 K. RESULTS

AND DISCUSSION

Examples of the dependence of the degree of conversion of nitrogen oxide, X,, on residence time, CO concentration and temperature are given in Figures l-3. Also shown in Figures I and 2 are calculations of XNo based on the kinetic analysis of the data discussed below. As shown in Figure I, XNo increases with residence time, but the strongest effect is achieved by increasing the CO concentration, as is evident from Figures I and 3 and more directly from Figure 2. The influence of temperature (Figure 3), on the other hand, is rather weak, with a moderate increase in X,, up to - 1125 K and a slight decrease at higher temperatures.

by CO: A. Berger and G. Rotzoll

0.8 -

Figure 2 Conversion of NO vs. CO concentration at different residence , 0.5; 0, 0.7; 0, 1.0. Solid lines and other conditions as for times (s): ?? Figure I

0.8 -

.$ 0.6 -

?? 0 .

P

0

g! 0.4 -

?? A

A

*

A

’ 973

1023

E o.z-

0.01

A

A

A A

’ 1073

t

’ 1123

8

4

’ 1173

Temperature [K] Figure 3 Conversion of NO vs. temperature at different CO concentrations (symbols as in Figure 1). Initial mole fraction of NO 1000 ppmv, residence time 0.8 s

Kinetic analysis

A simple kinetic model for the heterogeneous reaction NO+CO+$N,+CO, could consist of the following two reaction steps in analogy to the proposed reaction on char surfaces mentioned above: NO+*+O-*+$N, o-*+co-+*+co, where * represents a reactive surface site. NO is first adsorbed on the surface and dissociates, whereupon the nitrogen atoms quickly recombine to molecular nitrogen and desorb from the surface. The overall reaction rate is determined by the second step, reaction of the adsorbed 0 atoms with a CO molecule from the gas phase to form CO, and a free surface site. Assuming Langmuir-type adsorption for NO and a low surface coverage, the following rate expression is obtained: -dC,,ldt=k.CNoC,,

I

8.

0.5

I.

0.6

1

0.7

I

I.

0.8

1.

0.9

L

I

1.0

Residence time [s] Figure 1 Conversion of NO vs. residence time at different CO concentrations (vol.%): A, 1; A, 1.5; 0, 2; ?? , 3; 0, 4; ?? , 5. Solid lines: calculated values based on Equation (1) with rate constant from Figure 6. Initial NO concentration 1000 ppmv, temperature 1073 K

(1) To compare the experimental data with model predictions, the reactor is treated as a pseudo-homogeneous plug-flow reactor with uniform temperature. The time t is therefore equivalent to the calculated residence time r. The agreement of the hypothetical rate expression, Equation (l), with the data was tested in two ways. Since CO is present in large excess, its concentration can be

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Kinetics of NO reduction

by CO: A. Berger and G. Rotzoll

assumed to be constant, so that Equation (1) becomes effectively first-order: - dC,o/dt = k’CNo with k’= kG,,

(2)

which in integrated form yields

ln( 1 - XNo) = k’*t

(3)

where X,, is the degree of conversion of NO. Plots of the left-hand side against t or r therefore should yield straight lines with gradients k’. Examination of the data in this way gave very good linear plots in most cases. An example is given in Figure 4. The reaction order with respect to CO can be checked by plotting the left-hand side of Equation (3) versus Cc, at constant residence time, as shown for example in Figure 5. Again, good linearity is obtained, indicating the usefulness of Equation (1). From the slope of plots like those in Figures 4 and 5, the rate constant k at five temperatures was obtained. These are plotted in Arrhenius form in Figure 6. If the value at the highest temperature is omitted, the following expression for k results from a straight-line fit: k=5.4+107exp(-23.3/RT)

cm3 s-r mol-’

with activation energy 23.3 kJ mol-‘. This expression for k is valid for S/V=23 cm-‘. For other experimental

14.8 0.85

h 0.90

I 0.95

I 1.00

1.05

lC!OO/r[K-l] Figure 6 Arrhenius plot of rate constants for reaction of NO with CO catalysed by quartz glass. Straight line is expression for k quoted in the text

situations the pre-exponential factor has to be converted depending on the actual value of S/K The deviation of the experimental k value at 1173 K from the above expression may be due to a change in reaction mechanism, but this hypothesis needs to be substantiated by more data at higher temperatures. Comparison of measured and calculated X,o values in Figures I and 2 shows that the agreement is generally good, further demonstrating the suitability of the kinetic expression, Equation (1).

-0.8

SigniJicance for$uidized-bed -1.2 % = -1.6 B

0.5

0.6

0.7

0.8

0.9

1.0

Residence time [s] Figure 4 Plot of left-hand side of Equation (3) vs. residence time at different CO concentrations (vol.%): 0, 3; 0, 4; 4. 5. Initial NO concentration 1000 ppmv, temperature 1073 K

Figure 5 Plot of left-hand side of Equation (3) vs. CO concentration at different residence times (s): ?? , 0.6; 0, 0.8; 0, 1.0. Initial NO concentration lOOOppmv, temperature 1073 K

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combustion

A result of this work is the fact that CO has to be present in very large excess over NO in order to lead to significant heterogeneous reaction on quartz. This corresponds very well with the observations made in a fluidized bed of quartz sand particles, where low NO concentrations in the bed were found to correlate with high CO-concentrations, w 3% 12. A quantitative calculation can now be made for the reduction of NO by CO in the sand bed on the assumption that sand and quartz glass have the same catalytic activity (see ref. 12 for details). As S/V for the sand bed of ref. 12 was w lOOcm_‘, the rate constant derived in this work has to be multiplied by 100/23x4 in order to apply to that condition. With this value of k, calculation gives X,,x99%, in good agreement with the measured concentration profiles. It appears that high concentrations of CO are a prerequisite for the reaction to contribute to NO removal. High CO concentrations may exist locally in the particulate phase because of slow mass transfer or poor mixing, or more uniformly in the first stage of staged combustion. The importance of sand as a catalyst in comparison with the other bed components such as char and calcined limestone depends on the specific composition of the bed material. Comparison with these components is hampered by several difficulties concerning the applicability of kinetic data to other experimental situations. Often, overall rate constants are derived without any indication of the available surface area of catalyst per unit volume or even the total amount of catalyst. With porous materials such as char and calcined

Kinetics of NO reduction

limestone, additional difficulties arise in the correct calculation of internal diffusion resistance. The rate expression most often used for the NO-CC&char reaction is that derived by Chan er ~1.~.Since it is based on experiments with graphite, however, it may not be readily applicable to char. Recently, Johnsson and Dam-Johansen’ derived an alternative expression for the reaction, based on experiments with different chars, which will be taken here for comparison with the sand-catalysed reaction. For bituminous coal char particles of size 0.13 mm they derived the following rate expression for NO removal (Tab/e 7 in ref. 9): -dC,,/dt

= 5.1.106 exp( -20900/T)C~~3C~$8 mmol kg- ’ s- 1

A comparison with the sand-catalysed reaction will be made on the following assumptions. The bed consists of sand of size 0.2-0.3 mm with 2 vol.% of coal char of apparent density N 1 g cm- 3 and size 0.13 mm, for which the above rate expression applies. The concentrations of NO and CO are 1000 ppmv and 1 vol.% respectively. The ratio of reaction rates rsand/rcharfor these assumptions is calculated as w 2 at 1023 K, 0.4 at 1123 K, and 0.1 at 1223 K. It thus appears that the reaction catalysed by sand when present in large excess in a fluidized bed may indeed make a significant contribution to NO removal, as well as that on the char, depending on the specific composition of the bed and the temperature. A quantitative comparison of sand and calcined limestone is not possible, since not enough details are given in ref. 10. However, it is clear from that work that the intrinsic catalytic activity of calcined limestone is much higher, since no reaction occurred over quartz sand under conditions in which calcined limestone catalysed the NO/CO reaction. In an actual fluidized bed the solids mixture composition and the particle size will decide the contribution of each component. Also of importance is a possible limitation of the reaction rate by internal diffusion resistance of the more active materials char and calcined limestone.

by CO: A. Berger and G. Rotzoll

CONCLUSIONS The reduction of NO by CO is catalysed by quartz glass surfaces and therefore also by the similar material sand. Kinetic analysis of the reaction leads to reaction orders of unity for both NO and CO. The temperature dependence of the reaction rate is weak. The contribution of sand to NO removal in comparison with other constituents of the bed depends on the solid mixture composition and the particle sizes. As shown above, examples can be found in which the rate of the sand-catalysed reaction is comparable with the rate of the reaction catalysed by char, assuming a typical mixture composition. The sand-catalysed reaction should therefore be incorporated into reaction models of fluidized-bed combustors.

REFERENCES 1 2

Furusawa, T., Kunii, D., Oguma, A. and Yamada. N. Itrt. Chrr?l. Bq. 1980, 20, 239 Levy, J. M., Chan, L. K.. Sarofim, A. F. and BeCr, J. M. In ‘Eighteenth Symposium (International) on Combustion’. The Combustion Institute, Pittsburgh, 1981, p. 1I I Furusawa, T., Tsunoda, M. and Kunii, D. Ani. Ckrtr. Sot. .S~xlp. Ser. 1982, l%, 347 Chan, L. K., Sarofim, A. F. and Be&r, J. M. Cor~hust. Flrrrne 1983. 52, 37 de Soete, G. G. VDI-Berichte 1983. 498, 171 Furusawa, T., Tsunoda, M., Tsujimura, M. and Adschiri. T. Flrel 1985,64, 1306 Schuler. J., Baumann. H. and Klein, J. Er&/ Kohle, Erdycr.r Petrocllennl. 1988, 41, 296 Suuberg, E. M., Teng, H. and Calo, J. M. In ‘Twenty-third Symposium (International) on Combustion’, The Combustion Institute, Pittsburgh, 1990, p. 1199 Johnsson, J. E. and Dam-Johansen, K. In Proceedings of the 11th International Conference on Fluidized Bed Combustion (Ed. E. J. Anthony), ASME, New York, 1991, p. 1389 Tsujimura, M., Furusawa, T. and Kunii, D. J. Chem. Eng. Japan 1983, 16, 132 Furusawa, T., Koyama, M. and Tsujimura, M. Fuel 1985,64,413 Wittler, W., Schiitte, K., Rotzoll, G. and Schiigerl, K. Fuel 1988, 67,438 Hayhurst, A. N. Combust. Flame 1991, 85, 155

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